I’m trying to extend the range of the Hue motion sensor by installing a Hue smartplug half way between the sensor and the Hue bridge. I wonder if it is enough to connect the smartplug to the bridge, or do I also need to connect it separately to the sensor.It is powered, but doesn’t show up on the accessory list 
. I wonder why…
The smart plug should appear under your lights, so not under the smart controls / accessories. Just connecting the smart plug to the bridge should be good enough. You can connect it in Hue Essentials by going to the devices tab, tap the + button, tap Plug, select your Hue bridge, and search for lights.
Thank you for your tips 
Another question on the same topic. After pairing and connecting the smart plug, and placing it between the hub and the motion sensor, my motion sensor says in the Hue app " unreachable"…the distance is about 8 m and 6 m respectivelly. Few days ago it worked, now it doesn’t seem the sensor gets the signal Do I need to pair the sensor again to the smartplug ? Or the hub…??
It’s a but confusing 
A Hue plug/light has a range of about 10 meters. Maybe the motion sensor it a bit too far away (walls reduce the range).
The motion sensor shouldn’t have to be paired again, it should just pick up the connection automatically once it is in range and motion is detected. You could try pairing it again to the bridge, but I am afraid it will not fix the real issue.
I would suggest adding more lights/plugs or placing the motion sensor closer to the plug and see if that solves your issue.
I had the same issue. After working fine for many months all of a sudden the hub was having trouble communicating with my three motion sensors plus the only two other Hue devices I have. It was random though. Plus if I walked by a sensor it would go online for a while. I have a Hue button and if I walked it over to the hub it would work. But in my bedroom where it always did work it wouldn’t.
Changed batteries in everything even though they showed good.
I ordered a Hue smart plug to extend the range of my hub even though that didn’t make sense that it was a range issue as it seemed to be random sensors going offline.
WHAT FIXED IT
While waiting for the plug to arrive I found out how to fix this issue. I needed to go into the hub and change the Zigbee channel. It’s worked fine since.
Seems somehow something was interfering with its Zigbee channel. I did add a bunch of Aqara stuff a few months ago. It also uses a type of Zigbee so maybe that caused the interference. But odd that it didn’t happen sooner.
So today the Hue plug arrived so I put it in the middle of my home replacing the smart plug that was there.
I found this forum trying to find out if I needed to do anything extra with Hue smart plug to extend the Hue network or does it just do it automatically?